Impedance element with magnesium reaction terminal contact and method

ABSTRACT

An electrical impedance device is hermetically sealed within an encapsulating sleeve while external leads are simultaneously electrically connected thereto. The impedance element is disposed in an encapsulating sleeve which extends beyond the ends thereof. Magnesium is disposed within the ends of the sleeve, and one end of an external lead is placed in each end of the sleeve adjacent to the end of the impedance element. A bead of fusible material is disposed about the lead and is positioned adjacent to the end of the sleeve. Heat is applied to that portion of the assembly so formed including the bead and the end portion of the sleeve to cause the bead to form a hermetic seal with the lead and the sleeve and to cause the magnesium to vaporize and form a conductive cermet layer which forms an electrical connection between the lead and the electrical terminal of the impedance element. The magnesium also reacts with sleeve and bead to form a magnesium reaction product.

[ 51 May 7,1974

IMPEDANCE ELEMENT WITH MAGNESIUM REACTION TERMINAL CONTACT AND METHOD Robert D. DeLuca, Big Flats, N.Y.

Corning Glass Works, Corning, NY.

Filed: May 7, 1973 Appl. No.: 358,014

Inventor:

Assignee:

[5 6] References Cited UNITED STATES PATENTS 2/1935 Spencer [17/222 X 8/1957 Dobischer 117/222 X 2/1967 Griest 338/237 Primary Examiner-E. A. Goldberg Attorney, Agent, or Firm-William J. Simmons, Jr.

[5 7] ABSTRACT An electrical impedance device is hermetically sealed within an encapsulating sleeve while external leads are simultaneously electrically connected thereto. The impedance element is disposed in an encapsulating sleeve which extends beyond the ends thereof. Magnesium is disposed within the ends of the sleeve, and one end of an external lead is placed in each end of the sleeve adjacent to the end of the impedance element. A bead of fusible material is disposed about the lead and is positioned adjacent to the end of the sleeve. Heat is applied to that portion of the assembly so formed including the bead and the end portion of the sleeve to cause the bead to form a hermetic seal with the lead and the sleeve and to cause the magnesium to vaporize and form a conductive cermet layer which forms an electrical connection between the lead and the electrical terminalof the impedance element. The magnesium also reacts with sleeve and head to form a magnesium reaction product.

19 Claims, 7 Drawing Figures IMPEDANCE ELEMENT WITH MAGNESIUM REACTION TERMINAL CONTACT AND METHOD BACKGROUND OF THE INVENTION This invention relates to impedance devices and more particularly to a method of encapsulating or hermetically sealing impedance elements while simultaneously providing electrical contact between the electrical terminals of the impedance elements and their external leads. This invention further relates to the resulting structures.

Impedance devices such as resistors, capacitors, diodes, inductors and the like are usually encapsulated to provide the impedance element with a thermal barrier, to protect the element from attack by excessive moisture or corrosion, to electrically insulate the element from adjacent elements or, in certain applications, all of these functions may be served.

The prior art methods of electrical component encapsulation fall into two general catagories, the first of which is a potting method whereby the impedance element is coated with an appreciably thick layer of potting material. The potting material is usually in a fluid or semi-fluid state when initially applied to the element and is subsequently allowed to harden about the body of the element to provide the necessary protective coating. The other method is one wherein the impedance element is hermetically sealed in a container that may be either evacuated or filled with an inert atmosphere.

These known methods of electrical component fabrication usually call for the prior forming or manufacture of the complete impedance element, including the connection of external leads thereto, and then the subsequent step of either potting or sealing. An improvement over these methods is disclosed in US. Pat. Nos. 3,220,097 and 3,307,134 issued to E. M. Griest, wherein the leads are connected to the element while the element is being simultaneously hermetically sealed in a nonconductive sleeve. Briefly, the method disclosed in the aforementioned Griest patents is as follows. After a sleeve of fusible encapsulating material is disposed about the impedance element, both the ends of the impedance element and the ends of the leads to be connected thereto are provided with a fusible conductive ceramic frit. A fusible bead is disposed on the leads which are inserted into the ends of the encapsulating sleeve so that the frit on the ends of the leads contacts the frit on the ends of the element. Heat generated by a sealing flame is conducted through the sleeve, bead and lead, and the temperature of the frit becomes higher than the softening point thereof so that the frit on the element and that on the leads becomes fused while the bead fuses to the lead and sleeve. Although this method results in the simultaneous sealing of an impedance element in a hermetic container and connection of leads thereto, it requires that fusible conductive ceramic frit 'be initially applied to both the leads and the impedance element and then fired before the encapsulating step can begin.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a hermetically sealed impedance device that is noted by its ease of manufacture.

Another object of the.present invention is to provide a simple and economical method of forming hermetically sealed impedance devices.

A further object of this invention is to provide a method of forming an electrical component havingan evacuated, hermetically sealed enclosure.

Briefly, the present invention relates to an encapsulated impedance device comprising an impedance element having at least one electrical terminal having an end of an external lead disposed adjacent thereto. A sleeve of encapsulating material is disposed about the impedance element and the end of the external lead. A fusible element, having a coefficient of thermal expansion compatible with that of the encapsulating sleeve material, is disposed about the lead intermediate the ends thereof. The fusible element is fused to the lead and the sleeve to form a hermetic enclosure about the element. A cermet layer including magnesium and magnesium oxide, which is disposed upon the inner surfaces of the sleeve and fusible element and upon a portion of the impedance element and adjacent portion of the external lead, makes electrical contact between the impedance element and the external lead.

The encapsulated impedance device is formed in accordance with the following method. An impedance element having at least one electrical terminal at an end I thereof is provided, and a sleeve of fusible encapsulating material having at least one open end is disposed about the impedance element so that the open end extends beyond and provides access to the terminal. Magnesium is inserted into the open end of the sleeve, and an end of an external lead is placed into the open end of the sleeve. A fusible element is disposed about the lead intermediate the ends thereof. Heat is applied to that portion of the assembly so formed including the fusible element andthe adjacent portion of the sleeve to cause the fusible element to form a hermetic seal with the lead and the sleeve and to cause the magnesium to vaporize and form a conductive cermet layer which electrically connects the terminal and the lead.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. '1 is an exploded cross-sectional view of one end of a resistor blank indicating the components necessary to form an encapsulated resistor.

FIG. 2 is a cross-sectional view of one end of a completed resistor.

FIGS. 3 and 4 are cross-sectional views of the ends of impedance elements illustrating conductive'terminals which may be applied thereto prior to encapsulation.

FIGS. 5, 6 and 7 are cross-sectional views illustrating different forms in which magnesiums may be present during the encapsulation process. FIG. 5 shows powder or granules disposed on the resistor blank. In FIG. 6 a sheet of magnesium foil is affixed to an external lead, and in FIG. 7 the external lead is provided with magnesium paste prior to encapsulation.

DETAILED DESCRIPTION The process of the present invention and the product resulting therefrom will be described in terms of the components. As used herein, the term element or impedance element refers to the initially formed portion of an electrical impedance device or component prior to encapsulation and attachment of external leads thereto. The electrical terminals of such an element are either the actual ends of the impedance forming parts of the element or a conductive coating or end cap which is formed on the element in contact with the actual impedance forming part. For example, a resistor element, which is'often referred to as a resistor blank, comprises a substrate upon which is disposed an impedance forming part which may be a coil of wire, an electroconductive film or the like. Opposite ends of the film or wire may be considered to be the terminals of the element, or the terminals may be conductive films or caps in contact with the end portions of the film or wire. Theconnection of external leads to the electrical terminals of the element and the encapsulation thereof results in an impedance device or component.

Referring now to FIGS. 1 and 2 there is shown a resistor blank disposed within an insulating sleeve 12 of glass, glass-ceramic, ceramic or the like. Theinside diameter of sleeve 12 is very nearly the same as the outside diameter of blank 105 the spacing therebetween being preferably just sufficient to permit the blank to be easily inserted into the sleeve. One end of sleeve 12 may be fluted to facilitate insertion of blank 10. Blank 10 may consist of a substrate 14 having, for example, an electroconductive film 16 of tin and antimony oxides, nichrome, or the like deposited on the surface thereof. Substrate 14 is formed of a nonconductive material such as glass, ceramic or the like, and film 16 may be formed by any method known to those familiar with the art. A method of forming doped tin oxide films is disclosed in U.S. Pat. Nos. 2,564,706 and 2,564,707

issued in the name of John M. Mochel.

Sleeve 12 extends beyond the end of blank 10, and 1 the end of an external lead 24 is disposed near the end of blank 10 within the open end of sleeve 12. The end of lead 24 is preferably enlarged to provide improved electrical and mechanical contact with resistor blank 10.The end of lead 24 may be simply flattened or bent or a conductive disc 20, for example, may be welded or otherwise affixed to lead 24. Disposed between the end of disc 20 and substrate 14 is a magnesium wafer 18 which is utilized in the method of the present invention because of its low vaporization temperature and its reducing properties. The shape of wafer 10 preferably conforms to the geometry of the end of resistor blank 10.

Spaced about lead 24 is a toroidally shaped fusible element or bead 26, of glass or the like material, having a coefficient of thermal expansion compatible with that of the encapsulating sleeve material and lead 24. While bead 26 is herein depicted as toroidally shaped, it will be obvious to those skilled in the art that this head may be either toroidal or shaped like a washer, that is, it may be flat. In any event, the outside diameterof bead 26 should be only slightly smaller than the inside diameter of the encapsulating sleeve 12.

Hermetic sealing materials for use'as the sleeve 12 and bead 26 are well known in the art, and no attempt will be made herein to list specific compositions for such materials. Since glass sleeves and beads have been extensively utilized to provide excellent hermetic seals for electrical components, this material is preferred for use in the present invention. A sodalime-glass is often used as the sleeve while a lead glass is used for the fusible bead. Typical glass compositions for use in. the

manufacture of hermetically sealed electrical compo- Thomas. Since alkali metals react with some electroconductive resistor films, the use of alkali-free encapsulation glasses may be preferred for use with such films, such glasses being disclosed in some of the aforementioned patents.

Resistor blank 10 is cut to providethe required resistance, and, if necessary, film 16 may be appropriately spiralled to achieve higher resistances. The resistor blank shown in FIG. 1 may be encapsulated and provided with external leads in accordance with the method of the present invention, or it may be provided with conductive end portions prior to encapsulation by this method. As illustratedin FIG. 3 the ends of body 14 may be provided with a conductive film 38 in accordance with the teachings of U.S. Pat. No. 3,012,214 issued to R. W. Bronson et al. Alternatively, body 10 could be provided with conductive caps 42 as illustrated in FIG. 4. The embodiment illustrated in FIGS. 1 and 2 is preferred because it eliminates the steps required in the formation of the conductive end portions of FIGS. 3 and 4.

Whereas only one end of a resistor is shown in FIGS. 1 and 2, it is obvious that the same steps may be simul taneously or subsequently performed on the other end to form the completed resistor. It is preferred that the steps be simultaneously performed as hereinafter described. Resistor blank 10 is placed insleeve 12, and magnesium wafers 18 and conductive discs 20 are disposed at the ends thereof, beads 26 being disposed on leads 24 intermediate the ends thereof. Sufficient pressure is applied to lead wires 24 to keep conductive discs 20, magnesium wafers l8 and resistor blank 10 in intimate contact during the subsequent sealing process. An appropriate sealing flame is then applied to the juncture of sleeve 12 and bead 26 causing them to fuse together and form thejunction seal 28 shown in FIG. 2. Sleeve 12 extends beyond the end of blank 10 for a distancesufficlent to enable the formation of cavity' 22. Due to its proximity to the seal 28, the sealing-heat will melt and vaporize the magnesium metal, thereby forming a conductive film 30 which provides electrical contact between lead wire 24 and resistive film 16.

During the sealing operation heat generated by the flame is conducted through sleeve 12, bead 26, and

lead 24 to conductive disc 20, the temperature of which is thereby raised to a value which is sufficient to vaporize the adjacent magnesium wafer. When the glass sleeve and bead reach a sealing temperature of about 650700C, the temperature of the magnesium becomes high enough to cause vaporization, since the vapor pressure of magnesium is sufficiently high for it to sublime at 400C, and at 600C it has a very high vapor pressure, i.e., about 5 mm Hg. Moreover, magnesium is an extremelystrong reducing agent at temperatures above 600C, and the vapor thereof reacts with the oxygen in cavity 22 to form conductive cermet layers 30 and 32 containing MgO and Mg. Cermet layer I reaction products resulting from the reduction of those oxides. The remainder of cermet layer 30 can consist of magnesium intermetallic compounds and the metallic constituent of the oxides of the glass. For example, when magnesium reacts with SiO an extremely common glass constituent, the reaction initially produces a blue-grey conductive reaction product containing Mg, Mg Si, MgO and Si. Temperatures greater than about 500C cause the formation of the high temperature products MgO and Si. Regardless of whether all of the magnesium is consumed in the reaction, a conductive cermet results. If other oxides such as PbO, A1 K 0 or the like are present in the glass, intermetallic compounds of magnesium and the metals Pb, A] and K will be produced along with these latter mentioned metallic phases. The intermetallic compounds and metallic phases are usually electrical conductors, and the relative amounts of conducting and insulating phases determine the electrical resistivity of the resulting cermet. The composition of the cermet can be controlled by varying the amount of magnesium used, the temperature and time of reaction and the composition of the glass sleeve and bead. Due to the shortness of the path formed by that portion of cermet layer 30 which is disposed between film 16 and disc 20, the effect of the resistivity of layer 30 is negligible on the overall resistance of the encapsulated resistor. Since the magnesium reacts with the oxygen trapped in the cavity 22, an evacuated enclosure is formed without performing the sealing operation in a vacuum.

Magnesium may be disposed in the ends of the encapsulating sleeve in forms other than wafer 18 of FIG. 1. As shown in FIG. 5, wherein elements similar to those of FIG. 1 are indicated by primed reference numerals, a layer 48 of magnesium granules or powder may be disposed in one end of sleeve 12 which should be vertically disposed to prevent loss of magnesium. After one external lead is connected and one end of the resistor is sealed, the sleeve can be inverted and the second lead can be connected while the second end is sealed. FIGS. 6 and 7 show external leads having magnesium affixed thereto for ease of handling during the encapsulation process. As shown in FIG. 6, wherein elements similar to those of FIG. 1 are indicated by primed referencenumerals, a sheet 52 of magnesium foil is pierced by lead 54 to provide a combination that can be easily handled during the sealing and lead attachment process. As shown in FIG. 6, the end of lead 54 may directly contact the end of body 14. After the sealing process, a conductive cermet layer will extend from film 16 to lead 54 in a manner similar to that illustrated in FIG. 2, except that no conductive film will be disposed directly betweenlead 54 and substrate 14. The enlarged end 58 of external lead 60 of FIG. 7 is coated with a magnesium containing slurry or paste 62 which may consist of magnesium powder and a suitable vehicle such as water.

While the method of the present invention has been described in terms of resistance elements having electroconductive films, it is obvious that other types of resistorssuch as wire wound resistors may be employed equally as well. Also, other electrical components such as diodes or capacitive or inductive impedance elements can be hermetically sealed in accordance with the method of the present invention. In the case of a capacitor, the foil or plate ends may be provided with a conductive layer of the type shown in FIGS. 3 or 4.

Similarly, to encapsulate an inductive impedance, wire is wound about a form, and the wire ends and the form ends may be coated over with a conductive layer of the type shown in FIG. 4. Thereafter, the electrical connection of the terminal leads and hermetic sealing of the component is accomplished as hereinabove described in connection with the formation of encapsulated resistors.

I claim: 1. A method of forming an encapsulated impedance device comprising the steps of providing an impedance element having at least one electrical terminal at an end thereof, 7 providing a sleeve of fusible encapsulating material having at least one open end, disposing said sleeve about said impedance element so that said open end extends beyond and provides access to said terminal, disposing magnesium in said open end of said sleeve,

placing an end of an external lead into said open end of said sleeve,

disposing a fusible element about said lead intermediate the ends thereof, said fusible element having a coefficient of thermal expansion similar to that of said encapsulating sleeve material, and

applying heat, to that portion of the assembly so formed including said fusible element and the adjacent portion of said sleeve to cause said fusible element to form a hermetic seal with said lead and said sleeve, and to cause said magnesium to vaporize and form a conductive cermet layer extending between said electrical terminal and said lead.

2. A method in accordance with claim 1 further comprising the step of urging said end of said lead toward said impedance element during the step of applying heat.

3. A method in accordance with claim 1 wherein said fusible element is spaced from the end of said impe- 40 dance element so that the step of applying heat results in the formation of a cavity between said impedance element, sleeve and fusible element, and further causes said'magnesium to react with oxygen in said cavity to reduce the pressure therein.

4. A method of forming an encapsulated impedance device comprising the steps of providing an impedance element having electrical terminals at opposed ends thereof, disposing a sleeve of encapsulating material about said impedance element, the first and second open ends of said sleeve extending beyond the ends of said impedance element,

disposing magnesium in the first and-second ends of said sleeve,

inserting a first end of a first external first open end of said sleeve,

inserting a first end of a second external lead into said second end of said sleeve,

disposing a fusible element about each of said leads intermediate the ends thereof, said fusible elements having a coefficient of thermal expansion similar to that of said encapsulating sleeve material, and applying heat to those portions of the assembly so formed including said fusible elements and the adjacent ends of said sleeve to cause said fusible elements to form hermetic seals with their corresponding leads and with the corresponding ends of lead into said said, sleeve, and to cause said magnesium to vaporize and form a conductive cermet layer between said electrical terminals and said leads.

5. A method in accordance with claim 4 wherein said fusible elements are spaced from the ends of said impedance element so that the step of applying heat results in the formation of a cavity at each end of said impedance element between said impedance element, the adjacent end of said sleeve and said fusible element, and causes said magnesium to react with oxygen in said cavities to create a reduced pressure therein.

6. A method in accordance with claim 5 wherein said sleeve and said fusible elements are glass and where in the step of applying heat further causes said magnesium to react with the surfaces of said glass sleeve and fusible elements to reduce the same and form reaction products which constitute a part of said conductive cermet layer.

7. A method in accordance with claim 6 further comprising the steps of urging said first ends of said first and second leads toward said impedance element during the step of applying heat.

8. A method in accordance with claim 7 wherein the step of disposing magnesium in the open ends of said sleeve comprises disposing a magnesium wafer adjacent to each end of said impedance element.

9. A method of forming an encapsulated resistor comprising the steps of providing a resistor blank having a predetermined value of resistance and having electricalterminals at opposed ends thereof, disposing a sleeve of fusible encapsulating material about said resistor blank so that first and second open ends thereof extend beyond said blank, disposing a wafer of magnesium adjacent to each end of said resistor blank, providing first and second external leads, inserting a first end of said first lead into the first end of said sleeve, inserting the first end of said second lead into the second end of said sleeve,

disposing first and second fusible elements about said first and second'leads, respectively, intermediate the ends thereof, said fusible elements having a coefficient of thermal expansion similar to that of said encapsulating sleeve material, and

applying heat to those portions of the assembly so formed including said fusible elements and the adjacent ends of said sleeve to cause said fusible elements to form hermetic seals 'with their corresponding leads and with the corresponding ends of said sleeve, and to cause said magnesium to vaporize and form a conductive cermet layer between said electrical terminals and said leads.

l0. A'method in accordance with claim 9 wherein said sleeve and said fusible elements are glass and wherein said fusible elements are spaced from the ends of said impedance element so that the step of applying heat results in the formation of a cavity at each end of said blank between said blank,'the adjacent end of said sleeve and said fusible element and further causes said magnesium to react with oxygen in said cavities to create a reduced pressure therein and to react with the surfaces of said glass sleeve and fusible elements to reduce the same and form reaction products which constitute a part of said cermet layer.

11. A method in accordance with claim 10 wherein said first ends of said leads are provided with enlarged end portions, said method further comprising the step of urging the enlarged end portions of said first and second leads into engagement with the magnesium wafers adjacent thereto during the step of applying heat.

12. An encapsulated impedance device comprising an impedance element having at least one electrical terminal,

an external lead having an end thereof disposed adjacent to said electrical terminal,

a sleeve of encapsulating material disposed about said impedance element and said end of saidlead,

a fusible element disposed about said lead intermediate the ends thereof having a coefficient of thermal expansion compatible with that of said encapsulating sleeve material, said fusible element being fused to said lead and to the corresponding end of said sleeve, and

a conductive cermet layer disposed upon the inner surfaces of the end of said sleeve and said fusible element, said cermet layer including magnesium and magnesium oxide and providing an electrical connection between said electrical terminal and said lead..

13. An impedance element in accordance with claim 12 wherein a cavity is defined by the end of said impedance element and the inner surfaces of said fusible element and adjacent end of said sleeve, the pressure in said cavity being lower than ambient pressure.

14. An encapsulated impedance device in accordance with claim 13 wherein said sleeve andfusible element consist of glass and said cermet layer disposed upon said sleeve and fusible element includes reaction products that result from reduction of the surfaces of i a sleeve of encapsulating mate-rial disposed about said impedance element and said ends of said first and second leads,

first and second fusible elements disposed about said first and second leads, respectively, intermediate the ends thereof, the coefficient of thermal expansion of said fusible elements being compatible with that of said encapsulating sleeve material, each of said fusible elements being fused to its corresponding lead and to the corresponding end of said sleeve, and

.a conductive cermet layer disposed upon the inner surfaces of the ends of said sleeve and said fusible elements, said cermet layer including magnesium oxide and providing an electrical connection between said electrical terminals and associated I leads. I I 16. An encapsulated impedance device in accordance with claim 15 wherein cavities are defined by the ends of said impedance element and the inner surfaces of said fusible elements and adjacent ends of said sleeve, the pressure in said cavity being lower than ambient pressure.

17. An encapsulated impedance device in accordance with claim 16 wherein said sleeve and fusible elements consist of glass and said cermet layer disposed upon said sleeve and fusible elements includes reaction products that result from reduction of the surfaces of said glass sleeves and fusible elements by magnesium.

18. An encapsulated impedance device in accorenlarged end portions. 

1. A method of forming an encapsulated impedance device comprising the steps of providing an impedance element having at least one electrical terminal at an end thereof, providing a sleeve of fusible encapsulating material having at least one open end, disposing said sleeve about said impedance element so that said open end extends beyond and provides access to said terminal, disposing magnesium in said open end of said sleeve, placing an end of an external lead into said open end of said sleeve, disposing a fusible element about said lead intermediate the ends thereof, said fusible element having a coefficient of thermal expansion similar to that of said encapsulating sleeve material, and applying heat to that portion of the assembly so formed including said fusible element and the adjacent poRtion of said sleeve to cause said fusible element to form a hermetic seal with said lead and said sleeve, and to cause said magnesium to vaporize and form a conductive cermet layer extending between said electrical terminal and said lead.
 2. A method in accordance with claim 1 further comprising the step of urging said end of said lead toward said impedance element during the step of applying heat.
 3. A method in accordance with claim 1 wherein said fusible element is spaced from the end of said impedance element so that the step of applying heat results in the formation of a cavity between said impedance element, sleeve and fusible element, and further causes said magnesium to react with oxygen in said cavity to reduce the pressure therein.
 4. A method of forming an encapsulated impedance device comprising the steps of providing an impedance element having electrical terminals at opposed ends thereof, disposing a sleeve of encapsulating material about said impedance element, the first and second open ends of said sleeve extending beyond the ends of said impedance element, disposing magnesium in the first and second ends of said sleeve, inserting a first end of a first external lead into said first open end of said sleeve, inserting a first end of a second external lead into said second end of said sleeve, disposing a fusible element about each of said leads intermediate the ends thereof, said fusible elements having a coefficient of thermal expansion similar to that of said encapsulating sleeve material, and applying heat to those portions of the assembly so formed including said fusible elements and the adjacent ends of said sleeve to cause said fusible elements to form hermetic seals with their corresponding leads and with the corresponding ends of said sleeve, and to cause said magnesium to vaporize and form a conductive cermet layer between said electrical terminals and said leads.
 5. A method in accordance with claim 4 wherein said fusible elements are spaced from the ends of said impedance element so that the step of applying heat results in the formation of a cavity at each end of said impedance element between said impedance element, the adjacent end of said sleeve and said fusible element, and causes said magnesium to react with oxygen in said cavities to create a reduced pressure therein.
 6. A method in accordance with claim 5 wherein said sleeve and said fusible elements are glass and where in the step of applying heat further causes said magnesium to react with the surfaces of said glass sleeve and fusible elements to reduce the same and form reaction products which constitute a part of said conductive cermet layer.
 7. A method in accordance with claim 6 further comprising the steps of urging said first ends of said first and second leads toward said impedance element during the step of applying heat.
 8. A method in accordance with claim 7 wherein the step of disposing magnesium in the open ends of said sleeve comprises disposing a magnesium wafer adjacent to each end of said impedance element.
 9. A method of forming an encapsulated resistor comprising the steps of providing a resistor blank having a predetermined value of resistance and having electrical terminals at opposed ends thereof, disposing a sleeve of fusible encapsulating material about said resistor blank so that first and second open ends thereof extend beyond said blank, disposing a wafer of magnesium adjacent to each end of said resistor blank, providing first and second external leads, inserting a first end of said first lead into the first end of said sleeve, inserting the first end of said second lead into the second end of said sleeve, disposing first and second fusible elements about said first and second leads, respectively, intermediate the ends thereof, said fusible elements having a coefficient of thermal expansion similar to that of said encapsulating sleeve material, and applying heat to Those portions of the assembly so formed including said fusible elements and the adjacent ends of said sleeve to cause said fusible elements to form hermetic seals with their corresponding leads and with the corresponding ends of said sleeve, and to cause said magnesium to vaporize and form a conductive cermet layer between said electrical terminals and said leads.
 10. A method in accordance with claim 9 wherein said sleeve and said fusible elements are glass and wherein said fusible elements are spaced from the ends of said impedance element so that the step of applying heat results in the formation of a cavity at each end of said blank between said blank, the adjacent end of said sleeve and said fusible element and further causes said magnesium to react with oxygen in said cavities to create a reduced pressure therein and to react with the surfaces of said glass sleeve and fusible elements to reduce the same and form reaction products which constitute a part of said cermet layer.
 11. A method in accordance with claim 10 wherein said first ends of said leads are provided with enlarged end portions, said method further comprising the step of urging the enlarged end portions of said first and second leads into engagement with the magnesium wafers adjacent thereto during the step of applying heat.
 12. An encapsulated impedance device comprising an impedance element having at least one electrical terminal, an external lead having an end thereof disposed adjacent to said electrical terminal, a sleeve of encapsulating material disposed about said impedance element and said end of said lead, a fusible element disposed about said lead intermediate the ends thereof having a coefficient of thermal expansion compatible with that of said encapsulating sleeve material, said fusible element being fused to said lead and to the corresponding end of said sleeve, and a conductive cermet layer disposed upon the inner surfaces of the end of said sleeve and said fusible element, said cermet layer including magnesium and magnesium oxide and providing an electrical connection between said electrical terminal and said lead.
 13. An impedance element in accordance with claim 12 wherein a cavity is defined by the end of said impedance element and the inner surfaces of said fusible element and adjacent end of said sleeve, the pressure in said cavity being lower than ambient pressure.
 14. An encapsulated impedance device in accordance with claim 13 wherein said sleeve and fusible element consist of glass and said cermet layer disposed upon said sleeve and fusible element includes reaction products that result from reduction of the surfaces of said glass sleeve and fusible element by magnesium.
 15. An encapsulated impedance device comprising an impedance element having first and second electrical terminals at opposed ends thereof, a first external lead having an end thereof disposed adjacent to said first electrical terminal, a second external lead having an end thereof disposed adjacent to said second electrical terminal, a sleeve of encapsulating material disposed about said impedance element and said ends of said first and second leads, first and second fusible elements disposed about said first and second leads, respectively, intermediate the ends thereof, the coefficient of thermal expansion of said fusible elements being compatible with that of said encapsulating sleeve material, each of said fusible elements being fused to its corresponding lead and to the corresponding end of said sleeve, and a conductive cermet layer disposed upon the inner surfaces of the ends of said sleeve and said fusible elements, said cermet layer including magnesium oxide and providing an electrical connection between said electrical terminals and associated leads.
 16. An encapsulated impedance device in accordance with claim 15 wherein cavities are defined by the ends of said impedance element and the inner surfaces of said fusible elements and Adjacent ends of said sleeve, the pressure in said cavity being lower than ambient pressure.
 17. An encapsulated impedance device in accordance with claim 16 wherein said sleeve and fusible elements consist of glass and said cermet layer disposed upon said sleeve and fusible elements includes reaction products that result from reduction of the surfaces of said glass sleeves and fusible elements by magnesium.
 18. An encapsulated impedance device in accordance with claim 17 wherein said impedance element is a resistor blank having a predetermined value of resistance.
 19. An encapsulated impedance device in accordance with claim 18 wherein the ends of said first and second external leads that are disposed adjacent to said first and second electrical terminals are provided with enlarged end portions. 